GPU render turning out grey when using certain materials
I'm using AMD R9 280 graphics card and I had the same problem.

Reverting to older AMD drivers seems to fix the problem. I reverted from the newest drivers to 16.4.2 version and Blender (2.77) rendering on GPU works again!

You can find older driver versions here: http://support.amd.com/en-us/download/desktop/previous?os=Windows%207%20-%2064

Maybe you can revert to a bit newer drivers and the Blender will still work, I don't know, haven't tested it. I'm just really glad the GPU rendering works again and hopefully AMD fixes this issue asap.

Cheers!